1. Field of the Invention
This invention relates to a process for producing a printed wiring board, and more particularly to a process for producing a printed wiring board, that can form fine and highly precise wiring patterns and also can achieve a production cost reduction.
2. Description of the Prior Art
Printed wiring boards commonly comprise an insulating substrate made of glass fiber impregnated with an epoxy resin, and a copper conductive layer comprised of a copper wiring layer, an electrode and through holes, and are grouped into single-sided printed wiring boards, double-sided printed wiring boards, and multi-layer boards (multi-layer printed wiring boards). On these printed wiring boards, there is in recent years an increasing demand for making finer the wiring patterns formed of conductive layers.
Now, a production process hitherto commonly used for printed wiring boards will be described with reference to FIG. 10. A copper foil having been appropriately subjected to surface treatment such as surface roughening, corrosion prevention and thermal color-change prevention is laminated to the whole surface of an insulating substrate in the manner that the former's roughened surface comes in contact with the surface of the latter to obtain a printed wiring board material (a copper-clad laminate). Next, the surface of the copper foil is mechanically polished by buffing or the like and also chemically polished. Thereafter, to the surface thus polished, a photosensitive dry film is laminated to form a photosensitive resist layer. This photosensitive resist layer is patternwise exposed to light in a desired form, followed by development to pattern the photosensitive resist layer, and also the copper foil bared through this resist layer is subjected to wet etching to form a copper foil wiring layer. In such a conventional process, before forming the photosensitive resist layer, the copper foil surface is treated so as to be roughened and freshed to improve its adhesion to the photosensitive resist layer.
In the FIG. 10 flow chart, some steps are omitted, e.g., the step of forming through holes which is a subsequent step required when the printed wiring board is a multi-layer wiring board comprised of a plurality of inner-layer circuit boards, and the step of solder plating.
In this conventional process, however, the adhesion between the photosensitive resist layer and the copper foil surface can not be said to be satisfactory, and the process has the disadvantage that an ethcing solution tends to enter through a gap between the photosensitive resist layer and the copper foil surface to cause form disorder and pattern defects in the copper wiring layer formed after etching. Stated additionally, since the copper foil surface on which the photosensitive resist layer is a polished surface, it provides a light-reflecting plane inherent in metals, and this may also cause pattern form disorder when the photosensitive resist layer is patternwise exposed in a desired form.
These points will be explained with reference to FIG. 11. In FIG. 11, a copper layer (copper foil) 2a laminated to the surface of an insulating substrate 1 is a layer afterwards patternwise etched to form a wiring layer. In the step of exposure, the copper layer 2a is entirely covered with a photosensitive dry film (photosensitive resist) 3. This photosensitive dry film 3 is exposed to light through an exposure mask comprising a transparent substrate 5 having a negative type light-screening film 4 with a desired pattern, and thereby the dry film 3 is photo-cured in the desired pattern form. However, as stated above, the surface of the copper layer 2a forms a roughly glossy plane as a result of polishing, and hence, as shown in FIG. 11, incident light 6 reaches the surface of the copper layer 2a and irregularly reflects therefrom. In particular, the light like incident light 6a made incident from the part near to an end of the light-screening film 4 irregularly reflects from the surface of the copper layer 2a to cause disorder of photosensitive pattern form of the dry film 3 and also reflects from the surface of the light-screening film 4 of the exposure mask, where it successively repeats multiple reflection to increasingly cause disorder of the form of photosensitive regions of the dry film 3. This disorder of the form is presumed to be taken over also in development and etching to consequently make it difficult to form highly precise patterns as a wiring layer.
Meanwhile, in addition to such a disadvantage that the wiring layer can not be made into highly precise patterns, there is a problem that a phenomenon called "haloing" as explained below may occur when the multi-layer wiring boards are produced by this conventional process.
That is, the multi-layer wiring boards of this type are hitherto produced by laminating via adhesive insulating layers (prepregs) a plurality of inner-layer circuit boards each having a patterned wiring layer. A production process thereof will be schematically illustrated below with reference to FIGS. 12A to 12G. As shown in FIG. 12A, a plurality of inner-layer circuit boards a having copper wiring layers are put together layer-by-layer via adhesive insulating layers (prepregs) b together with outer-layer copper foils a.sub.1, and then a through hole c as shown in FIG. 12B is provided by a suitable boring means (e.g., drilling). Thereafter, this through hole c is chemically plated with copper in order to cover the inner wall of the through hole with a copper coating. Thus, a coating layer d as shown in FIG. 12C is formed on the laminate surface and the inner wall of the through hole c each.
Next, on the surface of this chemical copper coating layer d, a photoresist layer e is patternwise formed except part of the surrounding of the through hole c and the part where the wiring layer is formed (see FIG. 12D). Also, at the part bared to the surface from this photoresist layer e, a copper electrolytic plating layer f and a solder plating layer g are successively formed (see FIG. 12E). After the photoresist layer e has been removed, the chemical copper coating layer d is removed by etching as shown in FIG. 12F, using the copper electrolytic plating layer f and the solder plating layer g as masks. Subsequently, on these layers, a solder resist layer h is formed to produce a multi-layer wiring board i as shown in FIG. 12G. Such a process has been hither to employed.
In this process, when the above plurality of inner-layer circuit boards are laminated via the adhesive insulating layers (prepregs), the wiring layer provided on each inner-layer circuit board has no sufficient adhesion to the adhesive insulating layer since the former is made of copper, and the wiring layer tends with time to separate from the adhesive insulating layer. Accordingly, in the conventional process, the surface of the patternwise formed copper wiring layer is treated with, e.g., an alkaline aqueous sodium chlorite solution in which about 15 to 25 g/lit. of sodium hydroxide (NaOH) is dissolved, to form an oxide film comprised of black and acicular crystals CuO so that the acicular crystals oxide film is interposed between the wiring layer and the adhesive insulating layer to improve the bond strength of the former to the latter. Such a method, what is called black oxide treatment, has been employed. Employment of this method has brought about an improvement the bond strength between the wiring layer and the adhesive insulating layer as a whole. On the other hand, the oxide film (blackened film) formed of CuO has a resistance to alkali solutions but dissolves in acid relatively with ease. Hence, when the chemical copper plating of the above through hole is carried out using a hydrochloric acid type aqueous palladium-tin solution in order to impart thereto catalytic properties against the chemical copper plating, the cross sections of layers, bared from the inner wall surface of the through hole come in contact with the acidic aqueous palladium-tin solution and the oxide film dissolves at its part coming in contact therewith, so that the metal copper of the wiring layer is bared and a pink-colored ring comes to appear along the periphery of the through hole c as shown in FIG. 13., which is a phenomenon called "haloing". Such a disadvantage has been caused in this method.
Moreover, when after the treatment with the hydrochloric acid type aqueous palladium-tin solution the copper electrolytic plating layer is formed at the part bared from the photoresist layer, it is known to use as a plating solution an alkaline copper pyrrophosphate solution or an acidic copper sulfate solution. Since, however, the former copper pyrrophosphate solution may cause a problem on waste liquor disposal, the latter copper sulfate solution shows a tendency to be widely used. Hence, the oxide film may dissolve as a result of the electrolytic plating making use of such an acidic copper sulfate solution, often resulting in an increase in size of the pink-colored ring.
The width (L) of the pink-colored ring caused by the treatment with the hydrochloric acid type aqueous palladium-tin solution or the electrolytic plating making use of the copper sulfate solution tends to increase with a decrease in diameter of the through hole c. For example, it is 100 .mu.m or less in the case of a through hole of 0.8 mm diameter, while it is about 200 to 400 .mu.m in the case of a through hole of 0.4 mm or less diameter in some instances.
Once such a phenomenon has occurred, a gap is formed at the interface between the wiring layer and the adhesive insulating layer on the inner wall of the through hole, so that the bond strength between these layers may become lower to tend with time for the layers to separate and also the treating solution tends to remain in the gap during the production process, bringing about a problem of a lowering of reliability for multi-layer wiring boards.
Under such technical background, with regard to the former disadvantage pattern disorder of wiring layers, a method is proposed in which the copper layer surface is roughened in a smaller extent and also roughened uniformly so that the adhesion between the photosensitive resist layer and the copper layer surface can be improved and the wiring patterns precision can be made higher. More specifically, Japanese Patent Application Laid-open No 6- 45729 discloses such a method, according to which the copper layer surface of a copper-clad laminate is subjected to black oxide treatment to form a blackened film, and subsequently the surface layer is dissolved using hydrochloric acid to remove the metal oxide film so that the copper layer surface can be roughened in a smaller extent and also roughened uniformly, to thereby prevent ethcing solution entering in the gap between the photosensitive resist layer and the copper layer surface.
As for the latter disadvantage haloing phenomenon, which may occur only in the production of multi-layer wiring boards, Japanese Patent Application Laid-open No. 2-58898 discloses a method for preventing it, according to which part of the copper oxide film (blackened film) formed by black oxide treatment is dissolved using a 1/30N to 1/10N sulfuric acid solution to modify the surface state of the oxide film (blackened film) so that the haloing can be prevented.
In the method disclosed in Japanese Patent Application Laid-open No. 6-45729, however, since the blackened film formed on the copper layer surface is dissolved using a hydrochloric acid solution to remove the metal oxide film, the roughened surface of the copper layer still has a metallic gloss and hence the multiple reflection tends to occur between the light-screening film surface and the copper layer roughened surface. Thus, there is room for a further improvement in order to make the precision of wiring patterns higher. Also, when a hydrochloric acid solution with a lower concentration is used as the treating solution, copper chloride may be deposited on the treated surface, the surface of the blackened film, and tends to remain on the surface, so that it becomes impossible to well achieve fine surface roughness of the copper layer surface. The method also involves such a problem.
In the methods disclosed in Japanese Patent Applications Laid-open No. 2-58898 and No. 6-45729, hydrochloric acid or sulfuric acid is used in the treating solution for dissolving the surface of the blackened film. In relation thereto, these methods have an additional problem that, when a plurality of copper-clad laminates are continuously treated in the same bath holding the treating solution comprising hydrochloric acid or the like, the pH of the treating solution varies with an increase in the quantity of CuO having dissolved in the bath so that the copper layer surface may be treated in a non-uniform extent to cause a lowering of reliability of the resulting printed wiring boards.
Frequent change of the treating solution in the bath for a new treating solution may make it possible to treat the copper layer surface in a uniform extent, but employment of such a measure brings about the problems of a poor work efficiency and a relatively high treatment cost.